Analysis of Grain Boundaries, Twin Boundaries and Te Precipitates in Cd 1-x Zn x Te Grown by High-Pressure Bridgman Meth
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grain boundaries, affect the electrical transport properties inside the material.[3] The local effect of the grain boundaries, and the precipitates associated with them, may act to channel charge along certain paths inside the Cdl1 xZnxTe, and thus, create a device where the electrical transport properties are anisotropic. In order to gain insight on how these defect structures affect device performance, the structural and chemical nature of grain boundaries and twin boundaries in CdI xZnxTe are explored. Commercially available Cdl-xZnxTe is typically manufactured using a high-pressure Bridgman (HPB) technique.[4] In this method, a large over-pressure of an inert gas (typically Ar) is used to inhibit the evaporation of the charge material. Material prepared by this method has a polycrystalline microstructure with grains on the order of centimeters in size. In order to grow material by this method, several complications must be addressed. Perhaps one of the most challenging growth dilemmas is that of the retrograde solubility in the Cd-Te phase diagram.[5-7] As the Cdl-xZnxTe boule is cooled, Te and/or Cd precipitate out of the Cdl-.ZnxTe lattice. The chemistry of these precipitates is determined by whether the starting material is rich in Te or Cd. Precipitates tend to decorate internal boundaries, such as grain boundaries and twin boundaries inside Cdl-xZnxTe. The composition and distribution of the precipitates have been previously studied for CdTe and Cdl-xZnxTe materials grown by either HPB or Bridgman techniques.[2,8,9] Post-growth annealing treatments of the Cdl-xZnxTe boule has been used to reduce the number and concentration of precipitates, but the effect of annealing on the electrical properties of the material has yet to be determined.[10-12] In this study, Te precipitates have been investigated for their microstructure and crystallographic alignment with the surrounding Cdl-xZnxTe matrix. By analyzing the shape, alignment and internal microstructure of the Te precipitates, valuable insights 247 Mat. Res. Soc. Symp. Proc. Vol. 484 ©1998 Materials Research Society
into the mechanisms of their formation and their effect on the surrounding CdI xZnxTe lattice can be made. EXPERIMENTAL Polycrystalline Cdl-xZnxTe material, manufactured using HPB techniques, was obtained from commercial suppliers for analysis. This material was grown from a Te-rich charge with a nominal zinc concentration of 10%. An Ar over pressure was used to suppress the loss of Cd from the melt. Parameters related to the growth and cooling rates were not disclosed by the manufacturer. CdlxZnxTe samples were characterized using visible-light microscopy (VLM), transmission infrared-light microscopy (IR), scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (XEDS) and transmission electron microscopy (TEM). Prior to examining by VLM, IR and SEM, CdlxZnxTe samples were polished using one-micron alumina slurry and a felt cloth. Although this procedure produces a surface with noticeable scratches, no chemical alteration of the
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